The present invention relates to a wideband amplifier which serves to reduce a second order distortion particularly in UHF and VHF bands.
In receiving multi-channel signals in a wide range, these signals are subject to mutual interference, thereby disadvantageously causing distortions of the signals. Those distortions result from nonlinearity of an active device in a signal processing circuit. Though a number of high degree distortions may be brought about, the second degree and third degree distortions are the most significant ones caused in the active device used for signal processing. In particular, in a system for receiving signals in a VHF to UHF band such as a front-end amplifier of a cable television converter or a RF (Noise Frequency) modulator connected with a booster used in a video tape recorder, the second order distortion has to be carefully considered.
For wideband signal processing, a wideband bipolar transistor device or an FET (Field-Effect Transistor) consisting of a compound semiconductor may be employed. The bipolar transistor, however, has so inferior a noise figure that it gives rise to a large signal distortion. As a result, recently systems for processing signals in the VHF or UHF band often employ a compound semiconductor device such as a GaAs FET.
To describe such a wideband signal processing circuit, reference can be made to a wideband amplifier having a territory of a VHF to UHF band, in particular, a negative feedback amplifier consisting of a compound semiconductor such as a GaAs FET. FIG. 5 is an equivalent circuit of the GaAs wideband amplifier. FIG. 6 shows an example of the frequency dependence of the gain in this amplifier. It is assumed that a feedback resistance R.sub.f 12 is 300 .OMEGA., a gate bias resistance R.sub.G 13 is 10 k.OMEGA., FET 11 has a gate width of 800 .mu.m, a gate length of 1 .mu.m and a mutual conductance of 80 mS, and the consumed current I.sub.DD is about 30 mA. 14 denotes a choke coil C.sub.f for supplying bias current and 15 denotes a feedback capacitor for d.c. blocking.
When the wideband amplifier receives at an input terminal IN multi-channel signals over 64 to 80 channels (50 MH.sub.b to 500 MH.sub.b) in the VHF to UHF band, nonlinearity entailed in an element itself gives rise to second order and third order distortions of a signal. As noted above, the wideband amplifier used for the cable television converter requires such distortions to be strictly suppressed. The conventional GaAs FET, in essence, has a feature that only a small third order distortion is brought about. The present inventors, however, have discovered through their research that a large second order intermodulation distortion caused by the GaAs FET made it difficult for the FET to achieve the satisfactory efficiency for the intended use as an amplifier, in spite of the superior features of the FET in lowering noise.
To select an FET used for the amplifier, traditionally, high-frequency inspection has been manually done for large numbers of manufactured GaAs FETs one-by one in order to select an FET having a low noise and a small second order distortion. This was necessary because it has been so difficult to obtain a GaAs FET having a small second order distortion and a low noise.
The result was that on average just one satisfactory GaAs FET could be found among 70 FETs. This result indicates that it has been difficult to obtain a proper GaAs FET and the inspection has required a lot of labor and high cost, with the result that the GaAs FET amplifier has not been so easily manufactured.